The present invention relates to a new composite tube structure withstanding high pressures. Compared with tubes taught in the prior art the tubes of the invention have either a lower thickness and weight for equal service pressure, or a higher admissible pressure for equal thickness.
The invention further provides examples of economically optimized tubes.
By composite material should be understood a material formed from parallel fibers, such as type E or R glass fibers, carbon fibers, aramide fibers of Kevlar 29 or Kevlar 49 type (trademarks registered by Du Pont de Nemours) coated with a matrix such as a thermoplastic or heat hardenable material, for example, an epoxy resin. This matrix adheres to the fibers.
The invention applies, in particular, to the construction of tubes for transferring or storing fluids under pressure, such as water or hydrocarbons.
More particularly, the tubes of the invention are well adapted to be used in offshore oil working and search operations, for example, as safety lines for upgoing or downgoing standpipes connecting the bottom of the sea to a surface support such as a drilling or working platform, or such as a subsurface buoy. These standpipes are currently called risers. In the present text by composite monolayer should be understood as the juxtaposition and possibly superimposition of parallel fibers coated with a matrix. In the case of a tube, these fibers are wound at the same angle with respect to the axis of the tube.
By composite layer is meant either a monolayer, or the juxtaposition and possibly superimposition of fibers in two directions symmetrical with respect to an axis, these fibers being coated with a matrix. In the case of a tube, the fibers are wound at two opposite angles with respect to the axis of the tube.
By balanced composite layer is meant a layer comprising fibers disposed in two directions, with equal distribution of the fibers in these two directions.
The matrix adheres to the fibers. When a tube is formed from several composite layers, the matrix forms a continuous medium through these fibers to which it adheres, making the tube rigid. In the rest of this text, unless otherwise stated, the term layer will implicitly designate a composite layer.
The invention consists in winding substantially circumferentially, on a starting tube, a composite material having a circumferential modulus of elasticity higher than that of the internal, substantially circumferentially wound pressure resistant layers.
By circumferential modulus of elasticity of a composite layer wound on a tube is meant the modulus of elasticity (or Young's modulus) in a direction tangential to the layer considered, this tangent being situated in a plane perpendicular to the axis of the tube.
Composite tubes generally comprise superimposed fiber layers. As was mentioned above, in each layer these fibers are disposed at equal angles or angles symmetrical with respect to the axis of the tube and embedded in a matrix. This matrix adheres to the fibers of the different layers.
The invention applies more particularly, but not exclusively, to tubes whose layers, which only withstand, albeit completely, tractive forces are distinct from those which withstand substantially, albeit completely, pressure forces. The matrix coating the fibers of the different layers nevertheless forms a continuous medium through these fibers. The traction resistant layers comprise fibers wound at one or more small angles with respect to the axis of the tube. Similarly, the pressure resistant layers comprise fibers wound at high angles with respect to the axis of the tube. Without departing from the scope of the invention, instead of tubes comprising an external circumferential layer whose circumferential modulus of elasticity is greater than that of an internal layer, a pressure resistant casing may be formed from a composite material, such as a reservoir, comprising an external circumferential layer whose circumferential modulus of elasticity is greater than that of an internal layer.